1. Field of the Invention
The present invention relates to the technical field of analysis of blood circulating in an apparatus which provides for its purification, and which is commonly known as an artificial kidney. More particularly, the present invention relates to a method by means of which the sodium concentration in the blood may be determined, as well as to an artificial kidney incorporating means that provide for the implementation of this method.
2. Description of the Related Art
The purification of blood by means of an artificial kidney employs different modes of exchange between the blood and a purification fluid known as the dialysis fluid. These exchanges take place through a semipermeable membrane enabling the blood to be purified both by dialysis and by ultrafiltration.
The phenomenon of dialysis or diffusion is a consequence of the difference in concentration of solutes existing between the blood and the dialysis fluid. Substances present in the blood at a larger concentration than in the dialysis fluid tend to pass through the membrane of the exchanger until concentration equilibrium is established. Thus, if it is desired to eliminate certain substances present in excess in the blood, a dialysis fluid which is devoid thereof is circulated on the other side of the membrane. Conversely, if it is desired to enrich the blood with respect to a given substance, a dialysis fluid is chosen whose concentration of this substance is larger than that in the blood.
The second mode of purification used in an artificial kidney is ultrafiltration, which is the consequence of the pressure difference existing between the two fluids on each side of the membrane. In the case where the pressure of the blood is greater than the pressure of the dialysis fluid, a portion of the aqueous fraction of the blood passes through the semipermeable membrane and is then eliminated with the dialysis fluid.
This ultrafiltration phenomenon enables, in particular, the patient's excess water load to be abolished. In effect, in a chronic haemodialysed patient, the water which should normally be eliminated via the kidneys accumulates in the body between two haemodialysis sessions. This water excess is distributed between the intracellular compartment and the extracellular compartment including, in particular, the vascular system.
Although, during the haemodialysis session, the only compartment directly accessible is the vascular medium, it is not possible to abolish the patient's excess water load completely by simple ultrafiltration of the blood. This would, in effect, very quickly incur the risk of producing blood pressure drops in the patient, due to the decrease in blood volume. Moreover, if the aim is no more than to correct the extracellular hyperhydration, the well known syndrome of imbalance, which results, in particular, in headaches, nausea and cramp, is produced in the patient.
It is hence of importance, during the haemodialysis session, not only to achieve the desired weight loss, but also to monitor the water balance between the different compartments of the body.
One of the factors enabling this water balance to be monitored is the patient's blood sodium level. The doctor who supervises the progress of the haemodialysis session consequently needs to monitor the patient's blood sodium level so that the blood treatment conditions can then be modified if necessary.
In order to determine the value of a patient's blood sodium level, it is known, according to the prior art, to draw venous blood samples and then analyze them, by means of a sodium-specific electrode for example.
Although reliable, this method has, however, the drawback of being tedious and expensive on account, in particular, of the large number of calibrations required. It is not, moreover, advisable to draw too many blood samples from a haemodialysed patient who is, in general, already suffering from anaemia.
To remedy these drawbacks, Dr. Petitclerc proposes, in his thesis of 13th June 1985, entitled "Theoretical Approach and Clinical Application of Sodium Modeling During Haemodialysis," to replace the measurement of blood sodium level by a measurement of the conductivity of the dialysis fluid in equilibrium with the blood plasma. Dr. Petitclerc describes in his thesis a very good correlation existing between these two values.
In order to achieve equilibrium between the plasma and the dialysis fluid, Dr. Petitclerc proposes the recirculation of a small amount of dialysis fluid into the exchanger, until equilibrium, or an approximation thereof, is obtained. This recirculation stage lasts for approximately 10 minutes, and must be performed at each fresh measurement. During this recirculation stage, the ultrafiltration is maintained at its nominal rate.
This method for obtaining equilibrium between the dialysis fluid and the blood plasma has a very serious drawback. In effect, at each measurement stage, the purification of the blood by dialysis is very rapidly limited as a result of the recirculation of the dialysis fluid. Thus, in the case where it is desired to perform frequent determinations of the blood sodium level, the purification of the blood by dialysis becomes insufficient if the length of the haemodialysis session is not extended.
To enable good purification of the blood to be achieved by dialysis, it is hence necessary either to extend the length of the blood treatment or to limit the number of measurements performed. Whichever alternative is chosen, the solution is in no way satisfactory.
The object of the present invention is therefore to remedy the drawbacks of the prior art and to propose a method and an artificial kidney by means of which a patient's blood sodium level may be determined simply, rapidly, reliably and at low cost.
Another object of the present invention is to propose a method and an artificial kidney by means of which a patient's blood sodium level may be determined without the need to draw a sample of venous blood.
Another object of the present invention is to propose a method and an artificial kidney by means of which a patient's blood sodium level may be determined as frequenly as desired.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.